Secondary batteries commercially used at the present include nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, lithium secondary batteries or the like, among which the lithium secondary batteries are in the limelight due to their very low self-discharge rate, high energy density, and free charging/discharging since a memory effect does not substantially occur in comparison to nickel-based secondary batteries.
Such a lithium secondary battery mainly uses lithium-based oxide and a carbon material as a cathode active material and an anode active material, respectively. The lithium secondary battery includes an electrode assembly, which includes a cathode plate coated with the cathode active material, an anode plate coated with the anode active material, and a separator interposed therebetween, and an outer casing, i.e., a battery case, to accommodate with a hermetic seal the electrode assembly therein along with an electrolyte solution.
Generally, lithium secondary batteries may be classified, depending on a shape of a battery casing, into can shaped secondary batteries in which an electrode assembly is embedded in a metal casing and pouch-type secondary batteries in which an electrode assembly is embedded in a pouch of an aluminum laminate sheet.
Recently, secondary batteries have been extensively used in electric vehicles securing power using an internal combustion engine and/or an electric motor as well as in small devices such as portable electronic devices. The electric vehicles include a hybrid vehicle, a plug-in hybrid vehicle, a purely electric vehicle powered by only an electric motor and a battery without an internal combustion engine, and so forth.
For use in the electric vehicle, a number of secondary batteries are electrically connected to increase capacity and output. Especially for medium and large devices, a pouch-type secondary battery is mostly used due to its easy stacking.
However, the pouch-type secondary battery does not have high mechanical strength because it is generally packed with a battery case including a laminate sheet of aluminum and polymer resin. Thus, when a battery module includes multiple pouch-type secondary batteries, a cell cover is used to protect the secondary batteries from an external shock, etc., to prevent movement of the secondary batteries, and to facilitate stacking of the secondary batteries.
Meanwhile, if the temperature of a secondary battery rises higher than a proper temperature, the secondary battery may undergo performance deterioration, and in the worst case, may explode or catch fire. In particular, when a battery module is made by stacking multiple pouch-type secondary batteries, the temperature of the battery module may rise more quickly and drastically due to buildup of heat produced from the multiple secondary batteries in a small space. Moreover, a battery module included in a vehicle battery pack is likely to be often exposed to direct sunlight and to be in a high-temperature condition such as the summer season, a desert region, or the like.
Therefore, when a battery module includes multiple secondary batteries, it is very important to stably and effectively cool the secondary batteries. FIG. 1 illustrates a contact structure between a cell cover and a cooling plate and a heat transfer path according to a related art.
As illustrated in FIG. 1, a conventional battery module accommodates a secondary battery B and includes a cell cover 1 and a cooling plate 2 supporting the cell cover 1. The cooling plate 2 in the shape of an uneven plate includes a groove 3 on a top surface thereof, and the cell cover I is mounted in the groove 3.
Although not shown, a heat sink may be positioned under the cooling plate 2. Heat generated in the secondary battery B is transferred to the cooling plate 2 along a plate surface of the cell cover 1, and heat absorbed by the cooling plate 2 is transferred to the heat sink. The heat sink may be cooled by a coolant flowing along an internal flow path.
Meanwhile, in such a battery module, to improve cooling efficiency, it is necessary to secure a sufficient contact area between the cell cover 1 and the cooling plate 2, and a cell cover may not be closely mounted on a conventional cooling plate due to a manufacturing tolerance of the cooling plate. Especially when a surface of the groove 3 of the cooling plate 2 is not flat or even, a bottom of the cell cover 1 and the surface of the groove 3 may not completely contact each other. As a result, a gap is generated between the bottom of the cell cover I and the surface of the groove 3, increasing thermal contact resistance and thus lowering cooling efficiency.